This application claims the priority of German Application No. 199 12 140.0-21, filed Mar. 18, 1999, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a motor vehicle with an outer contour on which flow-influencing means are located to reduce the air resistance of the motor vehicle.
A motor vehicle of this type is known from U.S. Pat. No. 4,682,808. To reduce the air resistance of a motor vehicle with a fastback, preferably a commercial vehicle, various suggestions for mounting rigidly secured air guide elements in the trunk area are provided which influence the flow in the wake in the trunk area of the motor vehicle.
It is also known (German Patent Document DE 38 37 729 A1xe2x80x94corresponding to U.S. Pat. No. 5,407,245) to provide ejection and suction devices in the trunk area to reduce the flow resistance of a motor vehicle, with the ejection device producing a constant blast of air at high speed and the suction device producing constant suction of suitably deflected air in the eddy current area. The ejection and suction devices are combined with each other with one blower serving both to draw in air and to blow it out.
In an area in a different species, aircraft construction, it is known from U.S. Pat. No. 5,209,438 to provide oscillating movable disturbing elements in the wall boundary layer area of the flow around a wing in order to increase the lift of the wing. Alternatively, this patent provides for the oscillating application of disturbing elements to raise the divergence angle in a diffuser.
The goal of the invention is to provide a vehicle of the type recited at the outset that has an air resistance when driving that is reduced relative to the prior art.
This goal is achieved by the fact that means for influencing at least a coherent vortex structure in the vicinity of the outside flow around the vehicle have flow-influencing means. As a result, it is possible to considerably reduce the air resistance of the motor vehicle when driving. This measure also makes it possible to lower the fuel consumption of the vehicle. The invention is based on the fact that in blunt body areas of the motor vehicle, especially in the area of the trunk, but also in the rear areas of outside mirrors or similar blunt body areas, in which non-steady-state vortex structures appear in the separated outside flow area lead to a pressure drop downstream from the particular body area and thus lead to an increase in resistance.
In a rear area of the motor vehicle, the separated flow essentially forms so-called wake flow that is turbulent even at low speeds. Wake flow can also be spoken of in connection with body areas that are blunt at the rear. The more energetic the vortex structures in the wake, the greater the resultant air resistance. Such turbulent flows can have coherent structures in which the majority of the kinetic energy of the flow is concentrated. Such coherent vortex structures in the turbulent vortex flow lead to corresponding pressure drops. Thus, according to the invention, the coherent vortex structures are critical for the pressure drag and hence for a significant portion of the entire air resistance of a motor vehicle. The coherent vortex structures appear as a consequence of hydrodynamic instabilities in a statistically averaged basic flow. Appropriate means to influence these coherent vortex structures can alter the basic flow, so that the entire dynamics can be changed and a more energetically favorable flow around the motor vehicle can be achieved. This is turn results in a reduction of air resistance.
It has proven to be especially advantageous to introduce controlled time-dependent disturbances into the wake flow to reduce the intensity of the existing coherent vortex structures or to shift the development of these structures further downstream of the respective body area, especially the trunk. By deliberately introduced disturbances with the aid of the influencing means, coherent structures can appear at different dominant frequencies than in the natural wake. By introducing large amplitude disturbances the statistically averaged wake flow can be changed so that the average wake flow is more stable in the hydrodynamic sense and the creation of energetic coherent structures in the natural wake can be prevented or reduced or displaced in space.
Additionally or alternatively, it is advantageous to produce controlled time-dependent disturbances in the wall boundary layers in order to prevent separation of the flow in areas with positive pressure gradients in the flow direction. By introducing the disturbances, the energy exchange between the layers near the wall and the external flow can be considerably improved. Thus, adjacent flows with much greater pressure gradients in the main flow direction than in the undisturbed case can be produced. By using clear intakes with increased pressure gradients, greater pressure recovery and hence a reduced air resistance can be obtained up to the vehicle trunk. In addition, the statistically averaged wake flow that then appears in the wake is hydrodynamically less unstable, so that fewer intensive coherent structures appear in the wake and the air resistance is reduced further.
In certain preferred embodiments of the invention, the flow-influencing means are located movably in the vicinity of the outer contour and can be activated periodically with the aid of drive devices in order to introduce, time-dependent disturbances in the vicinity of the flow-influencing means into the relative wind flow. This design ensures further improved hydrodynamics and hence energetically more favorable flow around the motor vehicle. Since the flow-influencing means can be activated by the drive devices, the flow around the motor vehicle can be influenced actively. The provision of essentially stationary flow-influencing means on the other hand, as are known from the prior art, constitutes a passive measure to influence the flow around the motor vehicle which is not comparable with the solution according to the invention and with the design described.
In certain preferred embodiments of the invention, the drive devices include a control unit that changes the periodic activation of the flow-influencing means as a function of the speed of the motor vehicle. It is therefore possible to adjust the frequency of activation of the flow-influencing means to the respective flow rate of the relative wind and to the flow of air around the motor vehicle, so that air resistance can be reduced at all road speeds.
In certain preferred embodiments of the invention, the periodically activatable flow-influencing structure are combined with passive flow guide means in the vicinity of the outer contour. Passive flow guide structure as well as the periodically activatable flow-influencing means can be provided in both the trunk area and in the other areas of the outer contour of the motor vehicle. By a combination of active and passive measures to influence the flow of air around the vehicle, further reduction of the air resistance can be achieved.
In certain preferred embodiments of the invention, the periodically activatable flow-influencing devices are located in a trunk area of the motor vehicle to disturb wake flow. By introducing disturbances which are controlled and time-dependent into the wake flow, it is possible to reduce the intensity of existing vortex structures or to shift the development of such vortex structures further downstream of the motor vehicle trunk. It is also possible by introducing large-amplitude disturbances to change a statistically averaged wake flow so that the corresponding wake flow is more stable in the hydrodynamic sense and the development of high energy coherent structures in the natural wake is prevented, reduced, or spatially shifted.
In certain preferred embodiments of the invention, the periodically activatable flow-influencing means are in the outer contour areas located in front of the trunk area to disturb separation of the boundary layers of the flow around the vehicle. It is therefore possible to prevent separation of the flow around the vehicle in areas with a positive pressure gradient in the flow direction. By providing disturbances, the energy exchange between layers close to the wall and an external flow around the vehicle can be considerably improved. The influence on the wall boundary layers in the outer contour areas of the motor vehicle also guarantees change in the natural wake flow in the trunk area, with the modified wake flow having a higher stability hydrodynamically. The design therefore guarantees a reduction in air resistance by increasing the pressure in the main flow direction upstream of the end of the motor vehicle as well as a positive change in the wake flow so that the air resistance can be further reduced.
In certain preferred embodiments of the invention, a plurality of flow-influencing means are distributed over the rear area of the trunk that can be activated periodically in or out of phase with one another. Activation can then take place in such fashion that circulating disturbances can be produced.
In certain preferred embodiments of the invention, at least one movably mounted dimensionally stable air guide element is provided as the flow-influencing means. This is a design that can be realized simply.
In certain preferred embodiments of the invention, blowing and/or suction devices are provided as flow-influencing means whose ejection and/or intake openings are integrated into the outer contour. These suction openings can have a number of different shapes, especially circular or angular openings or narrow slots. These openings can also be covered by membranes so that the ejection and/or intake devices do not use air flows directly to produce disturbances but only for bellows-like actuation and movement of correspondingly flexible membranes. The membranes however can also be made permeable to air so that air flows result that can be blown out and drawn in.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.